1. Field of the Invention
The present invention relates to a charging roll for use in an image forming apparatus such as an electrophotographic copier, printer, or facsimile machine. In particular, the invention is concerned with such a charging roll for charging an image bearing medium such as a photoconductive or photosensitive medium used in electrophotography, and a dielectric medium used in electrostatic recording.
2. Description of the Related Art
In an image forming apparatus such as electrophotographic copier, printer, or facsimile machine, a so-called roll charging method has been widely adopted, in which an image bearing medium such as a photosensitive drum and a charging roll are mutually rotated, while the image bearing medium is held in contact with an outer circumferential surface of the charging roll, thereby charging the surface of the image bearing medium.
As charging rolls for use in the roll charging method, which is a kind of contact charging method, charging rolls with various structures have been conventionally suggested and used. Examples of conventional charging rolls include a charging roll with a structure in which a conductive elastic layer formed of a rubber layer having a low hardness is provided around a conductive shaft (metal core), and furthermore, a resistance adjusting layer and a protective layer are sequentially laminated on an outer circumferential surface of the conductive elastic layer as needed.
The conductive elastic layer of the charging roll having the above-described structure is conventionally formed of a rubber composition containing natural rubber or various synthetic rubbers with various additives including an electron-conductive agent such as carbon black. Specifically, with such a rubber composition being used as a molding material, the conductive elastic layer is obtained by a process including the steps of forming an unvulcanized (non-cross-linked) rubber composition layer having a predetermined thickness around the shaft according to various molding method, and vulcanizing or cross-linking the rubber composition layer.
The conductive elastic layer described above is required to be excellent in resistance to permanent set and charging uniformity and also required to have the lowest possible hardness, so as to allow the charging roll to efficiently charge the surface of the image bearing medium and to surely contact with the image bearing medium.
In recent years, a rubber composition including an ion-conductive rubber such as an epichlorohydrin rubber, as a rubber component, has come into use in the production of the conductive elastic layer of the charging roll. When molding is performed according to a conventional injection molding method by using such a rubber composition, the resultant molded body (an unvulcanized (non-cross-linked) rubber composition layer) may suffer from problems such as a poor surface property. To avoid the problems, an extrusion molding method has been widely adopted when molding a rubber composition including an ion-conductive rubber as a rubber component. In addition, in the above-described rubber composition, an inorganic filler such as calcium carbonate or silica is generally included in order to ensure moldability in extrusion molding and to facilitate grinding of the final vulcanized product (cross-linked product), as disclosed in Japanese Patent No. 3724465.
However, in the conductive elastic layer obtained by performing the extrusion molding and vulcanizing (cross-linking) the rubber composition that includes an inorganic filler such as calcium carbonate or silica, there is a problem in which the presence of the filler inevitably increases the hardness of the conductive elastic layer, and ultimately, it increases the hardness of the entire charging roll. In order to solve this problem, a rubber composition including liquid polymer have been used to form a conductive elastic layer, for example. However, in recent years, there is demand for miniaturization and higher-performance of image forming apparatuses such as electrophotographic copiers, and consequently, the charging roll are also required to have more improved characteristics. As a result, it is desired to develop a charging roll that has a low hardness with excellent resistance to permanent set and that is also capable of advantageously achieving required conductivity.